should have_many().through gives undefined method class_name' for nil:NilClass`

[marnen]

I've just run into a problem with shoulda-matchers 2.7.0 and 2.8.0, Rails 4.2, and Ruby 2.2.0 on Mac OS X 10.10. The complete application is at https://github.com/marnen/duckbill/tree/shoulda-matchers-issue , if you need to see the code in context.

Here's the relevant code:

# user.rb
has_many :clients
has_many :projects, through: :clients

# user_spec.rb
it { should have_many :clients }
it { should have_many(:projects).through :clients }

I would expect both specs to pass, but only the first one does. The second one gives the following error:

Failures:

  1) User should have many projects through clients
     Failure/Error: it {  should have_many(:projects).through :clients }
     NoMethodError:
       undefined method `class_name' for nil:NilClass
     # /Users/marnen/.rvm/gems/ruby-2.2.0/gems/activerecord-4.2.0/lib/active_record/reflection.rb:871:in `derive_class_name'
     # /Users/marnen/.rvm/gems/ruby-2.2.0/gems/activerecord-4.2.0/lib/active_record/reflection.rb:147:in `class_name'
     # /Users/marnen/.rvm/gems/ruby-2.2.0/gems/shoulda-matchers-2.8.0/lib/shoulda/matchers/active_record/association_matcher.rb:1067:in `rescue in class_exists?'
     # /Users/marnen/.rvm/gems/ruby-2.2.0/gems/shoulda-matchers-2.8.0/lib/shoulda/matchers/active_record/association_matcher.rb:1064:in `class_exists?'
     # /Users/marnen/.rvm/gems/ruby-2.2.0/gems/shoulda-matchers-2.8.0/lib/shoulda/matchers/active_record/association_matcher.rb:928:in `matches?'
     # ./spec/models/user_spec.rb:5:in `block (2 levels) in <top (required)>'

I have no idea what's going on here. Is this a Rails 4.2 incompatibility? Or something else? Help?

[lucianosousa]

Have you tried this?

it { should have_many(:projects).through(:clients) }

with parenthesis

[marnen]

I have no reason to believe that would make any difference at all, though I'll try it to be sure. Assuming I'm correct about that, what else would you advise?

[marnen]

I tried it with parentheses. Exact same issue.

[marnen]

Apparently, the problem was that while I had done User.has_many :projects, through: :clients, I had not declared Client.has_many :projects. Leaving this open because a more informative error message might be helpful.

[mcmire]

This will also occur if you don't define a proper belongs_to association in the model you're going through.

[maurogeorge]

@mcmire I started working on a fix to this on maurogeorge@48895ad.

It seens that AR treat this conditions and we can check for errors using the reflection.check_validity!, I wrote more useful info on the commit message before.

[mcmire]

Okay, that's a good start. I'd probably want a more specific error message in that case, but nice. Submit a PR and I'll take a closer look. (And btw I'm aware of your other PR's, I haven't had time lately to look at them but I'll get to them soon.)

[maurogeorge]

I have worked on this today @mcmire master...maurogeorge:have_many-through-error. But on the rails 4.1 I got a broken spec, I suspect that is a regression bug on Rails since it works on the 4.0.0, 4.0.1and 4.2.

I will take a look on it.

[maurogeorge]

@mcmire It was a bug on Rails 4.1 I send a patch and this was merged.

I will now take a look at this issue.

[mcmire]

Oh, very cool. Good to hear. Thanks for the work.

[ddavisgraphics]

I had the same issue with a reverse polymorphic association, the only error message was undefined method 'class_name'. Maybe that is expected, but even in the stack trace it was ambiguous and looked like it could have been a number of different errors. Is there a way to add a more concise error message?

Here is the code in case it helps.

class Field < ApplicationRecord
  belongs_to :form # this was the offender once added it worked
  belongs_to :field_types, polymorphic: true
end

class Form < ApplicationRecord
  has_many :fields, dependent: :destroy
end

class TypeA < ApplicationRecord
  has_one :form, through: :field
  has_one :field, as: :field_types
end

class TypeB < ApplicationRecord
  has_one :form, through: :field
  has_one :field, as: :field_types
end

[jmuheim]

I'm getting this error with the following models:

class Project < ApplicationRecord
  has_many :wcag_criterion_proxies, dependent: :destroy
  has_many :wcag_criteria, through: :wcag_criterion_proxies
end

class WcagCriterionProxy < ApplicationRecord
  belongs_to :project
  belongs_to :wcag_criterion
end

class WcagCriterion < WcagElement
  has_many :wcag_criterion_proxies, dependent: :destroy
  has_many :projects, through: :wcag_criterion_proxies
end

Can anybody see the problem? I don't see it.

[CharlieIGG]

Apparently, the problem was that while I had done User.has_many :projects, through: :clients, I had not declared Client.has_many :projects. Leaving this open because a more informative error message might be helpful.

This has also happened to me repeatedly, and each time I end up coming back to this thread (because my memory is bad). How can we add more significant error messages for this?

[mcmire]

#723 should fix this issue. It's just one of many, many PRs that we need to review. @guialbuk is this something you think you could take a look at?

[guialbuk]

For sure. I'm working on having that PR in a state that it can be merged, so we can fix that issue.

[mcmire]

A fix for this is now in master: c0a1578